The invention relates to methods and apparatus for performing precise laser interventions, and in particular those interventions relevant to improved methods and apparatus for precision laser surgery. In one preferred embodiment, the system of the invention is used for effecting precise laser eye surgery. In other embodiments the invention is applicable to non-surgical diagnostic procedures or non-medical procedures involving precision laser operations, such as industrial processes.
When performing laser interventions, whether in medical surgery, industrial processes, or otherwise, several fundamental considerations are common to most applications and will influence the viability and effectiveness of the intervention. To influence the outcome of the intervention, the present invention addresses both the technical innovations involved in an apparatus to facilitate precision laser interventions, and the methods by which a user of such apparatus can achieve a precise result.
The present invention addresses the following considerations: (1) how does the user identify a target for the laser intervention, (2) how does the user obtain information as to the location and other pertinent features of the target and its important surroundings, (3) how does the user lock onto that target so that the user has the assurance he is affecting the intended target, (4) how does the user localize the effect to the target site, (5) how does the user treat a large number of individual targets, whether continuously connected, piecewise connected, or disconnected, (6) how does the user assess the effect of the intervention, (7) how does the user correct errors committed either during the course of the intervention or as a result of previous interventions, (8) how does the user react to changing conditions during the course of the intervention to ensure the desired result, and (9) how is safety ensured consistent with U.S. Food and Drug Agency regulations for medical instruments and good commercial practice guidelines for industrial applications.
Of particular interest are medical interventions such as surgical procedures described by Sklar et. al. (U.S. patent applications Ser. Nos. 307,315 and 475,657, which are incorporated herein by reference). Although many different kinds of surgery fall within the scope of the present invention, attention is drawn to corneal refractive surgery in ophthalmology for the treatment of myopia, hyperopia, and astigmatism.
For corneal refractive surgery, the above nine considerations reduce to the following objectives (in accordance with the present invention described below): (1) identify the location on or in the cornea to be treated, (2) assure that the target is at the desired distance from the apparatus, determine the topography of the cornea, and determine the location of sensitive tissues to be avoided, (3) identify, quantify, and pursue the motion of suitable part of the cornea which can provide a reference landmark that will not be altered as a result of the surgical intervention and, likewise, the depth variations (for example, distance from the corneal surface to the front objective lens changing due to blood pressure pulses) of the corneal surface with respect to the apparatus such that said motions become transparent to the user of the apparatus, (4) provide a laser beam which can be focused onto the precise locations designated by the user such that peripheral damage is limited to within tolerable levels both surrounding the target site and along the laser beam path anterior and posterior to the target site, (5) provide a user interface wherein the user can either draw, adjust, or designate particular template patterns overlaid on a live video image of the cornea and provide the means for converting the template pattern into a sequence of automatic motion instructions which will traverse the laser beam to focus sequentially on a number of points in three dimensional space which will in turn replicate the designated template pattern into the corresponding surgical intervention, (6) assure that items (1)-(3) above can be performed continuously during the course of and subsequent to the surgery to monitor the evolution of the pertinent corneal surface and provide a means of accurate comparison between pre-operative and post-operative conditions, (7) ensure that the structural and physiological damage caused by the surgery to the patient is sufficiently small to permit continued interventions on the same eye, (8) automate the interaction between the various components so that their use is transparent to the user and so that sufficiently fast electronics accelerate completion of the surgical intervention within preselected error tolerances, and (9) provide dependable, fail-safe safety features of sufficiently short reaction times to prevent any chance of injury to sensitve corneal tissues. With these objectives fulfilled, the speed of surgery will no longer be limited by human perception delay and response times but by the capability of the apparatus to recognize changing patterns and adjust to the new conditions. Equally important, the accuracy of the surgery will not be constrained by the bounds of human dexterity, but by the mechanical resolution, precision, and response of advanced electro-optical and electromechanical systems.
There are a substantial number of different functions which the apparatus of the present invention addresses. Each of the complementary, and at times competing, functions requires its own technologies and corresponding subassemblies. The present invention describes how these various technologies integrate into a unified workstation to perform specific interventions most efficaciousely. For example, for corneal refractive surgery, as per (1) and (2) above, to identify the location to be treated on or in the cornea, the surgeon/user would use a combination of video imaging and automated diagnostic devices as described by Sklar et. al. (U.S. patent applications Ser. Nos. 307,315 and 475,657), depth ranging techniques as described by Fountain (U.S. patent application Ser. No. 655,919 filed Feb. 19, 1991), surface topographical techniques, as described by Sklar (U.S. Pat. No. 5,054,907) together with signal enhancement techniques for obtaining curvatures and charting the contours of the corneal surface as described by McMillan and Sklar (U.S. patent application Ser. No. 656,722 filed Feb. 19, 1991), profilimetry methods as disclosed by McMillan et. al. (copending U.S. patent application Ser. No. 07/842,879, referred to heretoafter as 266P, and entitled "Illumination of the Cornea for Profilometry," which was filed on the same date and assigned to the same party as the present application), image stabilization techniques as described by Fountain (U.S. patent application Ser. No. 655,919), which may all be combined using techniques as described by Sklar et. al. (U.S. patent applications Ser. Nos. 307,315 and 475,657). All of the above listed patent applications and the patent of Fountain (U.S. patent application Ser. No. 833,604 filed Feb. 11, 1992), are herein incorporated by reference.
Aspects of the above referenced disclosures are further used to provide means of satisfying the key aspects (3) through (9) noted above, such as verification of target distance from the apparatus, tracking the motion of the cornea in three dimensions, providing a laser whose parameters can be tuned to selectively generate photodisruption of tissues or photocoagulation as desired, automatically targeting and aiming the laser beam to precise locations, and supplying a surgeon/user with a relatively simple means of using the apparatus through a computer interface.
It is well known that visible light, which is passed without significant attenuation through most ophthalmic tissues, can be made to cause a plasma breakdown anywhere within eye tissue whenever the laser pulse can be focused to sufficiently high irradiance and fluence levels to support an avalanche process. The ensuing localized photodisruption is accomplished by using a strongly focussed laser beam such that only in the immediate focal zone is the electric field sufficiently strong to cause ionization and nowhere else. By using short pulses of controllably small laser energy, the damage region can be limited in a predictable manner while still guaranteeing the peak power necessary for localized ionization.
Furthermore, with lasers of increasingly higher repetition rate becoming available, the sometimes intricate patterns desired for a given surgical procedure can be accomplished much faster than the capabilities of a surgeon manually to aim and fire recursively. In prior systems and procedures, the surgeon would aim at a target, verify his alignment, and if the target had not moved, then fire the laser. He would then move on to the next target, and repeat the process. Thus, the limiting factor to the duration of the operation under these prior procedures was the surgeon's reaction time while he focussed on a target and the patient's movement while the surgeon found his target and reacted to the target recognition by firing the laser. In practice, a surgeon/user can manually observe, identify, move the laser focus to aim, and fire a laser at not more than two shots per second.
By contrast, a key object of the instrument and system of the present invention is to stabilize the motion of the patient by use of an automated target acquisition and tracking system which allows the surgeon to predetermine his firing pattern based on an image which is automatically stabilized over time. The only limitations in time with the system of the present invention relate to the repetition rate of the laser itself, and the ability of the tracking system to successfully stabilize the image to within the requisite error tolerances for safety and efficacy, while providing a means to automatically interrupt laser firing if the target is not found when a pulse is to be fired. Thus, where it would take several hours for a surgeon/user to execute a given number of shots manually (ignoring fatigue factors), only a few minutes would be required to perform the same procedure when authomatic verification of focal point position and target tracking are provided within the device.
It is an object of the present invention to accommodate the most demanding tolerances in laser surgery, particularly eye surgery but also for other medical specialties, through a method, apparatus and system for high-precision laser surgery which provides the surgeon "live" video images containing supporting diagnostic information about depth and position at which a surgical laser will be fired. In a computer, the full information content of a given signal is interpreted so as to provide this supporting diagnostic information, and the resulting accuracy achievable is within a few human cells or better.
The system, apparatus, and method of the present invention for precision laser surgery, particularly ophthalmic surgery, take a fully integrated approach based on a number of different instrumental functions combined within a single, fully automated unit. For example, previous conventional diagnostic instruments available to the ophthalmic surgeon have included several different apparatus designed to provide the surgeon/user limited measurement information regarding the cornea of the eye, such as the corneoscope, the keratometer, and the pachymeter. The corneoscope provides contour levels on the outer surface of the cornea, or corneal epithelial surface, derived, typically, from projected concentric illumination rings. The keratometer gives cross sectional curvatures of the epithelial surface layer resulting in an estimation of the diopter power of the front surface lens of the eye--the corneal epithelium surface. Only one group of points is examined, giving very limited information. Pachymeters are used to measure the central axial thicknesses of the cornea and anterior chamber.
The diagnostic functions fulfilled by these devices are instrumental to characterizing the subject tissue in sufficient detail to allow the surgeon to perform high precision ophthalmic surgery. Unfortunately, these and other similar instruments require considerable time to operate. Further, their use required near-total immobilization of the eye or, alternatively, the surgeon/user had to be satisfied with inherent inaccuracies; the immobilization methods thus determined the limitations on the accuracy and efficacy of eye surgery. Nor did the different apparatus lend themselves to being combined into one smoothly operating instrument.
For all of the above reasons, operation at time scales matched to the actual motions of the tissues targeted for therapy and/or limited by the fastest human response times to these motions ("real time") has not been possible with any of the conventional instruments used to date.
By contrast, the methods and apparatus disclosed herein, aim to incorporate a mapping and topography means for reconstructing the corneal surface shape and thickness across the entire cornea. It is furthermore within the scope of the present invention to provide such global measurements of the corneal refractive power without sacrificing local accuracies and while maintaining sufficient working distance between the eye and the the front optical element of the instrument (objective lens), said measurements to be executed on-line within time scales not limited to human response times. Most standard profilometry techniques were judged inadequate per the above requirements, requiring compromises in either acuuracies of the computed curvatures (such as, e.g., standard `k` readings of keratometers), speed and ease of operation (scanning confocal microscopes) or left no working distance for the ophthalmologist (corneoscopes and keratoscopes based on "placido disk" illumination patterns). It is therefore a key objective of the present invention to include a new topography assembly that can overcome the limitations of existing instruments while combining, on-line, and in a cost effective manner, many of the functions of conventional diagnostic instruments presently available to the surgeon, as an integral part of a complete surgical laser unit.
In one embodiment of the present invention, the corneal refractive power is measured using a unique projection and profilometry technique coupled with signal enhancement methods for surface reconstruction as disclosed by McMillan and Sklar in U.S. patent application Ser. No. 656,722 and further extended to larger corneal cross-sections via techniques described by McMillan et. al. in copending U.S. patent application Ser. No. 07/842,879, (per ref. 266P as cited above). In another embodiment, digitized slit lamp video images are used to measure the local radii of curvature across the entire corneal surface as well as the thickness of the cornea, with no built-in a-priori assumptions about the corneal shape. Both embodiments of the topography system benefit greatly from the availability of a 3D tracking capability contained within the apparatus. This feature allows elimination of many of the errors and ambiguities that tend to compromise the accuracy of even the best currently available instruments utilizing fine point edge extraction and advanced surface fitting techniques. With the computerized topographic methods of the present invention, surfaces can be reconstructed (and viewed in three dimensions) with accuracies that go well beyond the approximate photokeratometric and pachometry readings as advocated by L'Esperance (U.S. Pat. No. 4,669,466), or even the more sophisticated (but complex) corneal mapping methods as disclosed by Bille (U.S. patent application Ser. No. 07/494,683 now U.S. Pat. No. 5,062,702) and Baron (U.S. Pat. No. 4,761,071).
While tissue topography is a necessary diagnostic tool for measuring parameters instrumental to defining templates for the surgery (e.g., refractive power), such instrumentation is not condusive to use during surgery, but rather before and after surgery. Also, the information thus obtained is limited to those parameters characteristic of surface topography (such as radii of curvature of the anterior and/or posterior layers of the cornea or lens). Yet, in many cases, it is desirable to simultaneously image the target area and deposit laser energy at a specific location within the tissue itself. To allow reliable, on-line monitoring of a given surgical procedure, additional mapping and imaging means must therefore be incorporated. The imaging means is intended to record, in three dimentions, the location of significant features of the tissue to be operated upon, including features located well within the subject tissue. It is therefore another object of the present invention to provide continuously updated video images to be presented to the surgeon/user as the surgery progresses, said images to be produced in a cost effective manner yet compatible with high resolution and high magnification across a large field of view and at sufficiently low illumination levels to prevent any discomfort to the patient.
The imaging system, or the surgical microscope, requires viewing the reflected light from the cornea, which has two components: (a) specular (or mirror) reflection from a smooth surface, which returns the light at an angle opposite the angle of incidence about the normal from the surface and also preserves the polarization of the incident beam, and (b) diffuse reflection, in which light returned from a rough surface or inhomogeneous material is scattered in all directions and loses the polarization of the incident beam. No surface or material is perfectly smooth or rough; thus all reflected light has a specular and a scattered component. In the case of the cornea there is a strong specular reflection from the front surface/tear layer and weak scattered light from the cellular membranes below. Various standard `specular microscopes` have been used to suppress the front surface reflection. We have chosen a combination of techniques: some aim at observing the combined reflections without differentiating between specular or diffuse signals (for operations at or in immediate proximity to the surface of the cornea); in others the surface is illuminated with polarized light, with the reflected images then microscopically viewed through a crossed polarizer for operation within deeper layers, after selectively filtering the more anterior reflections. A rejection of the polarized component can thus be achieved, greatly enhancing resolution at low enough light levels to prevent any discomfort to the patient. In either embodiment, the imaging system contained within the apparatus of the invention represents a significant improvement over standard "slit lamp" microscopes such as are in use with most ophthalmic laser systems.
Other efforts at imaging the eye, such as performed with a Heidelberg Instruments Confocal Microscope, or as desribed by Bille (U.S. Pat. No. 4,579,430), either do not lend themselves to inclusion as part of an on-line, cost effective, integrated surgical system (for the former), or rely upon scanning techniques which do not capture an image of the eye at a given instant in time (for the latter). The method of the present invention benefits from having an instantaneous full image rather than a scanned image; for full efficacy, the method does, however, require that the targeted area be stabilized with respect to both the imaging and the laser focal region, so as to enhance the accuracy of laser deposition in tandem with the viewing sharpness.
Tracking is therefore considered a critical element of a system designed not only to diagnose, but to also select treatment, position the treatment beam and image the tissue simultaneousely with the treatment, while assuring safety at all times. In the case of corneal surgery, movements of the eye must be followed by a tracking system and, using dedicated microprocessors, at closed-loop refresh speeds surpassing those achievable by unaided human inspection, by at least an order of magnitude. Tracking by following the subject eye tissue, i.e., recognizing new locations of the same tissue and readjusting the imaging system and the surgical laser aim to the new location, assures that the laser, when firing through a prescribed pattern, will not deviate from the pattern an unacceptable distance. In preferred embodiments of the invention, this distance is held within 5 microns in all situations during ophthalmic surgery, which sets a margin of error for the procedure. It is possible that with future use and experimentation, it may be found that either more stringent or alternatively more lax displacement error tolerances are desirable to improve overall system performance.
Stabilization of a moving target requires defining the target, characterizing the motion of the target, and readjusting the aim of the apparatus of the present invention repeatedly in a closed-loop system. To meet accuracy goals also requires that the moving parts within the apparatus not contribute internal vibrations, overshoots, or other sources of positioning error which could cumulate to an error in excess of the prescribed mispositioning tolerances. There have been several previous attempts at achieving this result. Crane and Steele (Applied Optics, 24, p. 527, 1985) and Crane (U.S. Pat. No. 4,443,075) describe a dual Purkinje projection technique to compare the displacement of two different-order Purkinje projections over time, and a repositioning apparatus to adjust the isometric transformation corresponding to the motion. The tracking methods disclosed therein are based on a fundus illumination and monitoring device that aspires to distinguish translational from rotational eye movements, thus stabilizing an illuminating spot on the retina. However, localization of the Purkinje points can be influenced by transient relative motions between the various optical elements of the eye and may provide significantly fictitious position information for identifying the surface of the cornea. Motility studies as described by Katz et al. (American Journal of Ophthalmology, vol. 107, p. 356-360, "Slow Saccaddes in the Acquired Immunodeficiency Syndrome", April 1989) analyze the translations of an image on the retina from which the resulting coordinate transformation can be computed and galvanometric driven mirrors can be repositioned. In addition to the fictitious information discussed above due to relative motions between different layers of the eye, the galvanometer drives described by Katz usually are associated with considerable overshoot problems. Since saccaddes can be described as highly accelerated motions with constantly changing directions, overshoot errors can easily lead to unacceptable errors.
Bille et. al. (U.S. Pat. No. 4,848,340) describes a method of following a mark on the epithelial surface of the cornea, supposedly in proximity of the targeted surface material. However, in one of the uses of the present invention, a mark made on the epithelial surface would change its absolute location due to changes in the structure and shape of the material, caused by use of the instrument itself rather than by eye motions. Therefore, a target tracking and laser positioning mechanism that relies on a mark on the surface of the cornea in order to perform corneal surgery such as described by Bille's tracking method would be expected to lead to misdirected positioning of laser lesions below the surface when combined with any suitable focussed laser, as intended in one of the uses of the present invention. Moreover, one of the features of the present invention is to be able to perform surgery inside the cornea without having to incise the cornea. The main advantages of such a procedure are in avoiding exposure of the eye to infection and in minimizing patient discomfort. It would hence be counterproductive to mark the surface of the cornea for the purpose of following the motion of said mark. In another embodiment taught by Bille et. al., the tracking is based on a reference provided by either on the eye's symmetry axis, or the eye's visual axis, with an empirically determined offset between the two. Tracking is then accomplished by monitoring the reflection from the apex of the cornea, thus avoiding the need to mark the eye, and/or rely solely on patient fixation. However, with this technique, as in the preferred embodiment taught by Bille et. al., the tracking does not follow tissue features generally at the same location as the targeted surgical site on or inside the eye. Instead, Bille et. al.'s techniques track reference points that are, in all cases, separate, remote from and may be unrelated to the targeted surgical site. Such methods compromise accuracy of tracking in direct proportion to the degree of their remoteness relative to the surgical site. Therefore, they do not adequately provide for the fact that the eye is a living tissue, moving and changing shape to some extent constantly. Tracking a single point on the cornea, when the cornea itself actually shifts considerably on the eye, thus cannot be expected to reflect positional change of the targeted surgical site.
By contrast, in the preferred embodiment of the present invention the tracking information is obtained through means contiguous to the target region, which is mechanically and structurally considered as part of the cornea, but is unlikely to be affected by the course of the surgery and can thus provide a significant representation of non-surgically induced displacements. This is a critical feature of the tracking method disclosed herein, in that involuntary motions of the eye (such as are caused by blood vessel pulsing) can now be accurately accomodated, unlike techniques that rely on remote reference points.
The accuracy of the apparatus and system of the invention preferably is within 5 microns, as determined by a closed-loop system which incorporates actual measurement of the target position within the loop. (For example, a microstepper motor based assembly may have a single step resolution of 0.1 micron verified against a motor encoder, but thermal gradients in the slides may yield greater variations. Moreover, position of the slide can be verified via an independent optical encoder, but the random vibrations of the target can invalidate the relative accuracy of the motor.) Thus, the surgeon has knowledge of the shape of tissues within the field of view and the precise location of where he is aiming the instrument within those structures, to an accuracy of 5 microns. Such precision was not attainable in a systematic, predictable manner with any of the prior instruments or practices used. The present invention thus seeks to obviate the need for binocular vision used to obtain stereoptic images in some prior methods (see, e.g., Crane, U.S. Pat. No. 4,443,075).
In a preferred embodiment of the invention, the instrument also ensures that a laser pulse is fired only upon command of the computerized controller and after the system has verified that the tracking assembly is still locked onto the desired location, that the energy being emitted by the laser falls within prescribed error tolerances, and that the aiming and focussing mechanisms have reached their requested settings. There is no need for a separate aiming beam. In one embodiment of the present system, the method of parallax ranging is implemented to map out surfaces posterior to the cornea, but preceding actual treatment.
Safety is a very important consideration with laser surgery. In prior surgical systems and procedures, some safety shut off procedures for laser firing have depended upon human reaction time, such as the use of a surgeon's foot pedal for disabling the instrument when a situation arises which would make firing unsafe. In ophthalmology, some instruments have relied as a safety feature on a pressure sensor located where the patient's forehead normally rests during surgery. If insufficient pressure were detected by the sensor, the instrument would be disabled from firing.
Such prior safety systems have inherently had slow reaction times, and have not been able to react quickly enough to all of the various problems which can arise during a firing sequence. This is a critical concern in ophthalmic surgery, especially where specific surgical procedures are to be performed near sensitive non-regenerative tissues such as the corneal endothelium layer and the optic nerve. In contrast, the target capture and tracking system of the present invention makes available a new and highly dependable safety system. If for any reason, either prior to or during a given surgical procedure, the tracking system loses its target, the laser is disabled from firing. Various options are available for blocking emission from the apparatus once the tracking assembly has verified the loss of a tracking signal.
No previous surgical laser system has employed the efficacious combination of features as disclosed herein. For example, in previous art, Bille et. al. (U.S. Pat. No. 4,848,340) and Crane (U.S. Pat. No. 4,443,075) taught tracking techniques to follow tissue movements which might occur during surgery, but did not teach simultaneous 3D imaging within the tissue to monitor the effects of surgery on the tissue and provide requiste safety margins; L'Esperance (U.S. Pat. Nos. 4,669,466 and 4,665,913) also did not suggest any aspects of 3D imaging, teaching only laser surgery on the anterior surface of the cornea; Bille (U.S. Pat. No. 4,579,430) shows a retina scanner but does not teach simultaneous tracking. Bille et. al. (U.S. Pat. No. 4,881,808) teach an imaging system and incorporate a tracker and a beam guidance system by reference (per U.S. Pat. Nos. 4,848,340 and 4,901,718, respectively) but fail to address the very difficult challenges involved in achieving a smooth combination of all these aspects into a single surgical laser unit with built-in high reliability features. By contrast, it is the unique integration of several such diverse aspects (including mapping, imaging, tracking, precision laser cutting and user interface), precisely yet inexpensively, into a fully automated workstation, the uses of which are transparent to the user, that is the main subject of the present invention. The methods and apparatus disclosed herein are thus expected to enhance the capabilities of a surgeon/user in accomplishing increasingly more precise surgical interventions in a faster and more predictable manner. Enhanced safety is expected to be a natural outcome of the methods and apparatus taught herein in that the surgery will be performed without many of the risks associated with competing methods and apparatus such as described by L'Esperance (U.S. Pat. Nos. 4,669,466 and 4,665,913), Srinivasian (U.S. Pat. No. 4,784,135), Bille et. al. (U.S. Pat. No. 4,848,340, 4,881,808 and 4,907,586), Frankhauser (U.S. Pat. No. 4,391,275), Aron-Rosa (U.S. Pat. No. 4,309,998), Crane (U.S. Pat. No. 4,443,075) or others.